The commercially available Focus is a dedicated ultra-high-resolution small-animal 3D positron emission tomograph (PET) (Concorde Microsystems) employing lutetium ortho-oxysilicate (LSO) detector blocks. The LSO blocks in the Focus are unique in that the spacing between adjacent crystal elements (approximately 1x1 mm) is significantly narrower than in other systems, yielding significantly improved spatial resolution approximately- 1 mm (volume resolution: 1 mm3 - 10-fold better than current-generation microPETs). We have had 3 years experience with Concorde's older-design R4 microPET -resulting in 23 publications and 25 abstracts. Although very useful, the R4 has serious limitations for quantification in mouse tumor models (xenograft, transgenic, and knock-out). Our microPET users will thus greatly benefit from the state-of-the-art capabilities offered by the Focus - besides providing badly needed additional microPET imaging time for our many funded studies that involve imaging of radiotracers. We believe that we are well suited by experience and expertise to fully exploit the capabilities of the Focus. In addition, a well-established infrastructure is already in place to provide and coordinate the necessary logistical, technical, and scientific components critical to a successful small-animal imaging program. For example, the Cyclotron and Radiochemistry Core Laboratory includes our new (but fully operational) dual-beam, 19/9 Ebco cyclotron, producing a broad array of positron-emitting radionuclides (eg carbon-11, fluorine-18, gallium-66, yttium-86, iodine-124 etc) and radiotracers (e.g. C11-colchicine and -methionine, F18-FMiso, -FEAU, and -FDHT, Ga66-DOTATOC, Y86-DOTA-Biotin, and I124-FIAU and IAZG). Using such multiple radiotracers, regional tumor perfusion, metabolism, receptor and antigen levels, hypoxia, and gene expression can be assessed serially by microPET in the same animal - prior to, during, and after treatment, including the many novel drug, radiation, and radionuclide therapies being developed at MSKCC. In summary, the Animal Imaging Core Facility at MSKCC is being utilized by a continually expanding number of funded investigators and molecular imaging has emerged as a critical component of our research armamentarium. Acquisition of the state-of-the-art Focus will be an invaluable adjunct to resources already in place, and will further enhance our Center-wide, mission-critical capabilities in molecular imaging.